(A) Quantification of cardiomyocyte cross-sectional area stratified by disease duration reveals that patients diagnosed with heart failure < 1 year, 1–5 years, or < 5 years prior to tissue procurement displayed marked cardiomyocyte hypertrophy compared with donor controls. No significant differences were evident between disease duration groups. (B) Linear regression analysis describing the relationship between cardiomyocyte cross-sectional area and disease duration. (C) Quantification of total Picrosirius red staining stratified by disease duration reveals that patients diagnosed with heart failure < 1 year, 1–5 years, or < 5 years prior to tissue procurement displayed increased fibrosis compared with donor controls. (D) Linear regression analysis describing the relationship between total Picrosirius red staining and disease duration. (E and F) Quantification of interstitial and perivascular fibrosis stratified by disease duration demonstrates that myocardial fibrosis occurs independent of disease duration. No significant differences were evident between disease duration groups. (G and H) Quantification of CD34 staining stratified by disease duration demonstrates no relationship between microvascular density and disease duration. (I and J) Quantification of myocardial fibrosis (I) and cardiomyocyte cross-sectional area (J) stratified by disease duration reveals that children diagnosed with heart failure < 1 year or < 1 year prior to tissue procurement displayed no indistinguishable levels of myocardial fibrosis and cardiomyocyte hypertrophy compared with donor controls. No significant differences were evident between disease duration groups. *P < 0.05 compared with donor control. Mann Whitney U test (A, C, G) or χ² test (E and F).